Did a search and couldn't find the answer. I'm thinking of putting a 3rd generation Tri-Power on a Colt 6920 with the higher FSB. Will it co-witness with the factory mount or just be in the field of view. If not what mount will I need to buy? If not possible to co-witness will I need a riser to get the front sight out of the FOV?

Any standard mount will be fine. If you zero the tripower correctly, you will use the inside and outside of the chevron for man size targets at different ranges. Also works like a red dot as a CCO. Zero as instructed in the manual.